If my opponent plays Void Seeker and I Flash all my units away, are they forced to deal 4 damage to one of their own units at a battlefield?

Ruling: No, they are not forced to deal damage to their own units. Targeting happens before resolution, so when Void Seeker is played and a target is announced, you can React with Flash to remove the target, causing Void Seeker to fail.

Sequence:

• Opponent plays Void Seeker
• Opponent announces the target
• You React with Flash to remove your units
• Void Seeker fails to deal damage

Nuances:

• Void Seeker still draws 1 card even when it fails to deal damage